Though he lived in Maryville, Illinois, Fletcher began his playing career in the nearby St. Louis minor hockey system. After playing AAA hockey to increase his level of competition, Fletcher left high school his senior year to play for Sioux City of the USHL. He played three years for the Musketeers, his final year accumulating 12 goals and 31 assists -- good enough to be a 2nd Team USHL All-Star. He helped Sioux City to the league title in 2002. His first two years in Sioux City, his defense coach was Fred Harbinson, who left Sioux City for St. Cloud State University (SCSU) of the WCHA in Minnesota. Justin chose to play collegiately at SCSU not only because of his relationship with Harbinson, but the school was also losing some senior defensemen.

Fletcher played 29 games as a true freshman, netting 6 goals and 13 points, tops in SCSU's rookie class. He scored his first collegiate goal on a power play at North Dakota on Nov. 14, 2003. He'd continue to work his way up the depth chart and eventually become known as a high-character player and power play quarterback. He led SCSU defensemen in scoring his sophomore, junior, and senior seasons. Fletcher's plus-19 rating his junior year led the team. He was also named Academic All-WCHA after his junior year. His career high for points also came in his junior season, when he scored 6 goals and 21 assists for 27 points in 40 games. His plus-21 rating his senior year was best on the team.

For his SCSU career, the Real Estate major had 26 goals, 60 assist, 86 points, 200 PIM, and was plus-27 in 143 games. He currently stands 1st all-time among defensemen at SCSU in power play goals (18), and is second in assists and third in points. Among all skaters, he is 14th all-time on the school's record list for power play goals, 16th for assists, 7th for shots on goal (312), 13th for plus/minus, and 13th for PIM.

After finishing his career with SCSU, Fletcher, an undrafted free agent, was signed to a Amateur Tryout Contract by the Springfield Falcons of the AHL. Fletcher provided an immediate offensive boost to the struggling Falcons, finishing his brief stay in Springfield with three goals and an assist 10 games. The Lightning thought enough of Fletcher's game to sign him to a one year, two-way NHL contract in the summer of 2007.

Fletcher served as a fill-in for the Lightning at the 2007 Traverse City, Michigan prospects tournament, where he earned positive reviews from the scouting staff. Fletcher finished the tournament with one assist, a minus-1 rating, and 6 PIM as the Lightning went 0-4.